This invention relates to the field of protective circuitry for a thyristor and more particularly to protection from undesired breakover of the thyristor. This protection is provided by a minimum of two break over diodes, BOD, elements connected in series, whose combined threshold voltages are smaller than the thyristor's off-state break over voltage and which, having the same polarity as the thyristor, are connected between the thyristor's anode and its gate.
The undesired ignition of thyristors can occur if the anode-cathode voltage either exceeds the off-state break over voltage or if this voltage increases with a very short rise time (e.g., 2 to 3 microseconds). Such undesired ignitions generally occur in an uncontrolled fashion at a localized random spot along the cathode junction surface with the possibility of leading to a destruction of the thyristor due to the high dissipation losses encountered in such a switch-on.
The undesired ignition is transformed into a desired self-ignition by a corresponding control pulse at the thyristor gate. This prevents destruction of the thyristor in the case of voltages exceeding the rated breakover voltage or the critical off-state voltage increase rate.
One type of protection circuit is known from DE-OS No. 24 44 910, which operates both statically as well as dynamically, i.e., which provides protection both against exceeding the off-state break over voltage as well as exceeding permissible dv/dt values. It is, however, a more complex design using transistor circuitry.
Another type of break over diode protective circuit is known from the Cigre report 14-05/19, 1976. It is installed in the Canadian high-voltage d.c. transmission system (HVDC) designated as "Nelson River." In said circuit the protection against break over ignition of the thyristors when exceeding the off-state break over voltage is provided using high-voltage sweep diodes, also called BOD elements, which are connected to the anode and gate of the thyristor to be protected. In this arrangement, several BOD elements are connected in series with parallel-connected high-ohmic resistances for voltage stabilization. The advantage of this protective circuitry with BOD elements is that when exceeding the off-state break over voltage or threshold voltage of the BOD elements, a precise control pulse is transmitted to the thyristor, whereby additionally during the ignition delay time, of the BOD elements and particularly of the thyristors to be protected, in conjunction with the protective reactance coil or the transformer leakage inductance, a voltage limitation is established. An ignition in the event of exceeding the permissible dv/dt values for the thyristor is not, however, initiated, that is to say, the Nelson River protective circuitry does not operate in a dynamic fashion. Accordingly, it will be appreciated that it would be highly desirable to provide a protection circuit with the simplicity and performance of the "Nelson River" protective circuit yet has the dynamic performance against fast rise time voltage increases.
It is an object of this invention to design a protective circuit having the simplicity and speed of the break over diode type, which also operates dynamically, i.e., initiates a protective ignition when exceeding the permissible dv/dt values.